Energy absorption materials have been widely used to protect people and goods from damaging impacts and forces. Energy absorption materials can be dividied into two categories: those without truss architecture, and those with truss architecture. The former category includes cellular materials such as metallic or polymeric closed or open cell foams, crushed honeycombs, or other commercial materials such as Skydex™. The latter category includes micro-truss structures composed of solid or hollow members (struts, trusses, or lattices) with constant architectural parameters such as unit cell size, radius, length, or angles of each member, through the thickness direction of the structure. For the former category, the cellular materials dissipate kinetic energy associated with impact via elastic and/or inelastic deformation. The compression response of foam and pre-crushed honeycomb materials approaches an ideal response (as shown in FIG. 1, which will be described in more detail afterwards), but the ability of these materials is limited either by the low densification strain in foams or the low load-bearing capability in pre-crushed honeycombs. In either case, although the response characteristics are ideal, the performance of the material suffers due to non-ideal spatial arrangement of the microstructures.
Previous materials with truss or lattice architecture have constant architectural parameters through the thickness direction, i.e., the energy absorbing direction of the truss or lattice structure. The high structural symmetry and lack of disconnected internal members lead to simultaneous buckling and a sharp loss of load transfer capability as shown in FIG. 2. This reduces the energy absorption efficiency of the material as the stress level associated with compaction drops well below the peak value.
Turning now to the behavior of a given energy absorption material during impact or compression, architected materials composed of truss- or beam-like elements experience collapse mechanisms in which the incoming energy or external work is absorbed in three stages: initial buckling, compaction at a constant or near-constant stress plateau, and ultimately full densification. FIG. 1 is a schematic plot illustrating the ideal behavior of an energy absorption material. The initial response of the material is a compressive strain that changes linearly with the compressive stress corresponding to the material response prior to the onset of buckling or plasticity. After reaching a peak stress 101, the ideal material response switches from the linear elastic stage to a constant stress plateau stage 102, where the force transmitted through the material remains uniform and constant, until the material reaches a densification stage in which the strain increases rapidly, linearly or non-linearly, with the stress again. The strain corresponding to the transition point from the plateau stress to the densification stage is identified as the densification strain 103. The maximum possible volumetric energy absoprtion for a given material is calculated as the product of the peak stress 101 with 100% strain. However, actual architected materials will have deviation from the ideal response and lead to a loss of absorption efficiency. FIG. 2 illustrates the typical behavior of a lattice or truss structure with high structural symmetry and internal connectivity. Here, after reaching a peak stress 201 at the onset of buckling, rather than staying at the peak stress level, the compressive stress drops to a lower plateau stress 202. This is believed to be due to the fact that the onset of buckling at a single point in a structure with high structural symmetry and internal connectivity will trigger buckling throughout the structure, which leads to an instantaneous loss of load-carrying capability and reduced energy absorption efficiency. In such a case, the densification strain 203 is defined as the strain level corresponding to the interception of a horizontal line at the peak stress value with the stress-strain curve. The actual volumetric energy absorbed is calculated as the area under the stress-strain curve between 0% strain and the densification strain. The energy absorption efficiency of such a material is calculated as ratio of the actual volumetric energy absorbed to the maximum possible volumetric energy absorption.
Therefore, there is still a demand for lattice architectures with the inherent structural and low mass benefits, yet with improved energy absorption response.